This invention relates to a coil antenna used for transmitting and/or for receiving radio signals within a low or medium frequency band. Specifically, the target frequencies of the radio signals range from 10 kHz to 5 MHz.
There have been used or proposed various kinds of apparatuses, systems, or terminals, which transmit and/or receive radio signals of low or medium frequencies. A typical, well-known system is an AM (amplitude modulation) radio system. A relatively new system is a radio controlled timepiece, especially, a radio controlled wristwatch. Other relatively new system is an immobilizer for vehicle, a remote keyless entry system for vehicle or for house, or an RFID system. For more is information about a radio controlled wristwatch, see U.S. Pat. No. 6,134,188, which is incorporated herein by reference in its entirety. For more information about a remote keyless entry system for vehicle, see U.S. Pat. No. 6,677,851, which is incorporated herein by reference in its entirety.
An important component common to the above-mentioned apparatuses or the like is an antenna, especially, a coil antenna which comprises a magnetic core and a coil wound around the magnetic core. For example, an already-existing magnetic core for coil antenna is made of a sintered ferrite core or a laminated core consisting of amorphous metal sheets. The former is easily breakable and does not have flexibility on design because of its hardness. The latter is not easily machinable and is expensive so that its manufacturing cost becomes high.
An improved coil antenna is disclosed in JP-A 2001-337181, which is incorporated herein by reference in its entirety. The disclosed coil antenna is used for a radio controlled timepiece or wristwatch and has a core comprised of powder particles or flakes of ferrite or metal and a plastic binder agent. The core may hold another harder core, such as a sintered ferrite core or a laminated core made of amorphous metal sheets. The core comprised of JP-A 2001-337181 possesses high impact resistance because of its softness and can be readily formed with low cost.